Trees are one of the many organisms with a metabolism driven by cyclic dynamics. Like animals that hibernate over the winter and resume activity in the spring, trees have also developed mechanisms that allow them to survive the harsh Canadian winter temperatures and start anew in spring. But how do trees detect the arrival of winter and, more importantly, how do they know to wake up once spring has arrived?
The annual growth cycle of trees in Canada is influenced by the changing of seasons. In fact, the determining factors in a tree’s growth cycle are photoperiod length (the period during which the tree performs photosynthesis) and temperature. This growth cycle is regulated in turn by the growing season and dormancy period cycle. In Canada, trees experience extreme fluctuations in temperature over the course of a year—ranging from +30 to -40°C—which is why they have developed specialized defense mechanisms to survive winter temperatures. The process begins at the end of summer, when the photoperiod grows shorter. During this time, trees progressively send less energy towards the crown, preferring to focus on building up energy reserves in the roots. These energy reserves act as a natural antifreeze, protecting the roots from the cold over the winter. At this stage, trees gradually enter the dormancy period. Metabolic activity is minimal, though not completely suspended. At the same time, apical meristems on the tips of branches are encapsulated in protective scales to form buds. These apical meristems allow for the growth of new leaves in the spring. Once the defense mechanisms are in place, the tree is ready to tolerate the winter climate until spring returns.
For many, spring is a joyous season: a sign that days are growing longer, and that summer will soon be on our doorstep. For trees, the arrival of spring signals the beginning of the growing season. As the photoperiod lengthens and temperatures rise, trees receive the signal to break dormancy. It is time for defense mechanisms to cease and bud break to begin. This period is also when the flow of tree sap is reactivated, and when buds shed their protective scales so that the apical meristems can eventually produce new leaves and flowers for the tree. Tree growth is most pronounced in the spring. The lengthening photoperiod, rise in temperatures, and ground thaw all provide the tree with an abundance of water and solar energy, allowing it to grow vigorously. This time of year also delights producers and lovers of maple syrup alike. When the tree’s sap-transporting system is reactivated, the sweet sap, which flows from the reserves in the roots to the crown of the tree to feed the buds, is collected. Once collected, this sugar water can be transformed into the delicious and much beloved maple syrup.
Photoperiod- and temperature-mediated control of growth cessation and dormancy in trees: a molecular perspective, Jay P Maurya, Rishikesh P Bhalerao, Annals of Botany, Volume 120, Issue 3, September 2017, Pages 351–360.